There are parts of the transmission system for which most AM stations have no backup and those are the phaser, antenna tuning units and the towers themselves as well as related coaxial cable, switches and the like. All of these need maintenance, and all of them can deteriorate or become defective. So going off the air for one or more nights of maintenance is nothing unusual. A very few stations have an auxiliary antenna or an auxiliary transmitter site (although these are quite common with FMs).
The FCC generally would not even know about that sort of thing unless the station found a need to operate under an STA with parameters at variance, which requires a filing.
David,
I think you may have misunderstood me. I was trying to get across that I agreed with another poster who mentioned that WRKO didn't seem to go out of its way to tell their audience that they could be heard on 100.7 HD-2.
I do fully understand a directional AM cannot afford to duplicate its antenna farm in any way, shape, or form.